Given its pretty clear the EU has no intention of giving the UK an easy time in 'reset' negotiations (and who can be surprised by that after the toxic politics of Brexit), the idea of moving towards negotiations to (re)join the custom union (and eventually the EU) assumes the EU would have the UK back on terms that are (politically) acceptable to the UK.
We can argue around this for ever, but my general feeling is (now at least) the gap is too wide for a negotiated settlement.